The present invention relates to a color image processing apparatus and more particularly to a color image processing apparatus for adjusting a pulse width and the like by using a patch image.
In a so-called image forming apparatus such as a copying machine, a printer or the like, recently, color printing has become a mainstream and strict color density characteristics have correspondingly been required for a printing color density. For example, a color copying machine performs a color converting process for converting RGB image data input from a scanner into colorant primaries CMY image data to be output by using a colorant by means of a printer. In order to duplicate an image on a document by faithful color reproducing properties, it is necessary to properly adjust the characteristics of color conversion (color conversion parameter, color conversion table).
In order to faithfully duplicate, however, it is necessary to properly adjust gradation characteristics as well as color conversion. For example, image data input from a scanner has received distortion for respective RGB in a process of passing through a sensor and a filter. Also in a printer, moreover, gradation and colors which are to be output are sometimes varied by an environment (temperature and humidity), a deterioration with age and a manufacturing solid-state difference even if the same image data are given. In particular, gradation characteristics on the printer side are sensitively changed by various factors. Therefore, it is necessary to provide a mechanism for optimizing output gradation characteristics and color conversion characteristics on demand in an apparatus in order to keep good image reproducing properties.
As a method for obtaining the color conversion parameter, there has conventionally been known a method for reading a sample color patch output from an apparatus itself to collect RGB-CMY data pairs of patches having plural colors, thereby obtaining a coefficient of a conversion polynomial from the RGB into the CMY by using a method of least squares. This method regards the characteristics of an image processing section other than the color conversion as a black box and is based on the thought that an image can be ideally reproduced faithfully only by the inverse correction of the black box to be carried out by the color conversion.
On the other hand, there has also been known a technique for fixedly setting the color conversion parameter to adjust the output gradation characteristics by the same method as described above on a certain assumption. More specifically, a sample gradation patch output from an apparatus itself is read to collect Cxe2x80x2-C, Mxe2x80x2-M and Yxe2x80x2-Y data pairs of a plurality of gradation patches, thereby obtaining a coefficient of the gradation correcting equation of each of C, M and Y by using a method of least squares.
In practice, however, it is hard to obtain the image outputs of sufficient gradation and color reproducing properties even if the above-mentioned adjusting method is used. The reason is as follows. An image process from a scanner to a printer is constituted by inking, a gradation process (pseudo halftone process), a screen process and the like in addition to the color conversion. In these blocks, nonlinear distortions having various properties are to image data, respectively.
In the conventional image forming apparatus, accordingly, there has been a problem in that it is hard to collectively generate parameter for correcting the nonlinear distortions such as the color conversion process, the inking process, the gradation process, the screen process and the like.
It is an object of the present invention to provide a color image processing apparatus for stepwise determining a plurality of parameters for correcting a distortion, thereby obtaining an output of a color image having excellent gradations and color producing properties.
The present invention provides a color image processing apparatus using a gradation image recording method by pulse width modulation, comprising pulse width selection table determining means for determining a pulse width selection table for pulse width modulation, gradation correction table determining means for determining a gradation correction table based on the pulse width selection table determined by the pulse width selection table determining means, color conversion table determining for determining a color conversion table based on the gradation correction table determined by the gradation correction table determining means, and color image processing means for correcting a given color image signal and outputting the corrected color image signal based on the color conversion table determined by the color conversion table determining means, the gradation correction table determined by the gradation correction table determining means and the pulse width selection table determined by the pulse width selection table determining means.
According to the present invention, as described above, it is possible to provide a color image processing apparatus in which when the color conversion table, the gradation correction table and the pulse width selection table are to be determined, the pulse width selection table having the lowest degree of freedom is first determined, and subsequently, the tables having a lower degree of freedom are sequentially determined to obtain an optimum correction value, thereby outputting color images having excellent gradations and color reproduction.
Furthermore, the features of the present invention are specified by the claims by taking an example using a plurality of image patches to determine each table. Consequently, it is possible to provide a color image processing apparatus capable of performing excellent image correction in the same way.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.